The 60/40 portfolio is discussed like it's a real thing, when in truth it is really a figment in our collective memory. It describes modern asset allocation about as well as "Happy Days" did the 1950s or "Friends" the 1990s. (Great TV shows both — but a bit detached from the real world.)
60/40 has instead become a convenient straw man for highlighting the benefits of other, more sophisticated strategies: "X has higher returns!; Y has a higher Sharpe ratio!; Z delivers better diversification!" All well and good. But before we dismiss it as an artifact, consider that it offers a clear glimpse of the "genetic code" upon which most modern asset allocation is based, and which still exerts a pronounced anchoring effect on investors. Equities dominate given their high long-term return expectations but require diversification given their high volatility. Bonds provide a negatively correlated asset class with modest positive returns, making them an ideal diversifier. Returns, volatilities and correlations balance: Hence, 60/40.
Asset allocation sophisticates may scoff at the simplicity of this model, preferring instead to speak of risk factors, volatility regimes and conditional correlations. And though these latter-day innovations provide a richer and more nuanced picture of portfolio behavior, they are in some sense just old wine in new bottles. If the objective is high long-term returns, then just about any model will zero in on a portfolio with high equity-risk-factor exposure, diversified by duration ... not that different than the basic 60/40.